Tumor virus SV40 provides an opportunity for the study of the role of single genes in the initiation and maintenance of neoplastic cell transformation. Studies with temperature-sensitive mutants have identified the A gene that is required for both productive and transforming infection. In productive infection, the A gene regulates viral DNA replication and transcription. In transfroming infection, it is required to initiate and possibly to maintain the new growth chracteristics of cells. We have identified the A protein and are studying its function using nucleoprotein complexes. We are purifying wild-type and mutant A proteins to investigate their function in vitro. Studies with deletion mutants show that the A gene does not account for the entire early region of the genome and raise the possibility that a second early gene is also required for transformation. We are attempting to identify its gene product. BIBLIOGRAPHIC REFERENCES: Tegtmeyer, P., Rundell, K. and Collins, J.K., "Modification of Simian Virus 40 Protein A", J. Virol. 21, 647-657 (1977). Rundell, K., Tegtmeyer, P., Collins, J.K., Lai, C.-J., Nathans, D. and Ozer, H.L., "Identification of Simian Virus 40 Protein A", J. Virol. 21, 636-646 (1977).